PART I: In this part of the proposal the effects of Nitroprusside and Nitroglycerin on left ventricular function and the coronary vasculature will be studied under controlled hemodynamic conditions in normal and ischemic hearts. Regional coronary blood flow will be studied with radioactive microspheres. It is expected that the results of these experiments will contribute to a better understanding of the appropriate usage and predicted effects of these drugs in the clinical situation. PART II: Since we have previously shown that sequential imaging after an intravenous dose of Thallium-201 can differentiate between transient underperfusion or ischemia and myocardial infarction, this protocol will evaluate the ability to quantify redistribution of Thallium-201 to detect changes in regional myocardial perfusion induced by Nitroprusside and Nitroglycerin during ischemia and infarction in a canine model. PART III: The distribution of intracoronary Thallium-201 and Technetium-99m albumin microspheres will be compared with microspheres injected into the left atrium to evaluate the similarity of results obtained with the first two clinical agents to those obtained with standard experimental microspheres. In addition myocardial images will be made with Thallium-201 of ischemic myocardium with particular attention to segmental wall perfusion and motion.